1. Field of the Invention
The present invention relates to regulators (power processors) generally and more specifically to a feedback circuit having improved high-speed characteristics, a circuit particularly useful with a buck or a buck derived regulator.
2. Description of the Prior Art
The feedback employed in many regulators is less than ideal. Consider, for example, a basic prior-art-type buck regulator (power processor). Such a regulator includes a transistor functioning as a switch connected between a regulator-internal node and an input terminal, the terminal for connection to receive a regulator-driving voltage. The switch is driven so as to periodically couple the input-terminal regulator-driving voltage to the internal node to develop at the node a switched voltage which varies as a train of pulses. Also included is a filter having an inductor and capacitor. The inductor is connected between the internal node and an output terminal, the terminal for connection to a load. The capacitor is connected between the output terminal and a voltage-common (circuit ground) terminal. The filter averages the internal-node switched voltage to develop at the output terminal a voltage suitable for driving the load. To provide a path for inductor current when the switch is not coupling the input-terminal regulator-driving voltage to the internal node, a diode is included connected between the internal node and the common terminal.
Finally, the regulator includes an amplifier and an analog-to-discrete-interval (A/DI) converter, the combination connected to form a feedback path from the regulator-output terminal to the switch. The amplifier compares (at least) a scaled portion of the output-terminal load-driving voltage to (at least) a scaled portion of an (internally or externally generated) reference (control) voltage and amplifies the difference voltage to develop an amplified error voltage the level of which represents (in amplified form) the deviation from the ideal (error) of the level of the output-terminal load-driving voltage. The analog-to-discrete, interval converter drives the switch. Responsive to the level of the amplified error voltage, the analog-to-discrete-interval converter alters, as appropriate, the switch drive to adjust the internal-node-switched-voltage-pulse-train duty-factor (the pulse width (duration) and/or the pulse repetition rate) so as to correct the output-terminal-load-driving-voltage level.
The output-terminal load-driving voltage is less than ideal for feedback purposes. This is because the output-terminal load-driving voltage reflects the delay associated with the filter, a delay which constrains the feedback-loop bandwidth limiting the rate at which the feedback loop can respond to changes in, for example, the level of the input-terminal regulator-driving voltage.
To further exemplify the problem, consider that to reduce the voltage level of the switching ripple and noise (as a component of the output-terminal load-driving voltage with respect to its level as a component of the internal-node switched voltage) by more than a factor of 100 requires that the filter have a filter frequency of less than one tenth the switching-ripple frequency (pulse-train rate). Also, loop stability requires that the feedback-loop gain be reduced to unity at a frequency less than approximately one half the filter frequency, unless compensation is provided for (one or both of the two poles of) the filter. Thus, a buck regulator having a 48 Khz nominal pulse train rate, a 4.8 Khz filter frequency and a feedback loop the gain of which is reduced to unity at a frequency of 2400 Hz, would provide little reduction in the level of the 2400 Hz ripple component of an input-terminal regulator-driving voltage developed by full-wave rectifying power obtained from a three-phase 400 Hz main.
Of course, a higher feedback-loop unity-gain frequency may be employed if compensation is provided for the filter. Unfortunately, the component value stability, particularly that of the capacitor, limits the usefulness of this technique. Additionally, some improvement has been achieved by feeding internal-node information (forward) around the filter to the comparator such as by means of another inductor winding for sampling the inductor voltage drop.
Recognizing these limitations (in a 1971 National Aeronautics and Space Administration publication entitled "Power Processing" and designated NASA SP-244) Francisc C. Schwarz suggested employing for feedback purposes the internal-node witched voltage. In addition to employing the internal-node voltage in a new (Type 1) feedback loop, Schwarz suggested retaining in modified form the old (Type 0) feedback loop as an outer, secondary, loop to correct for residual, low-frequency, errors. For the outer, secondary, feedback loop, Schwarz suggested developing the amplified error voltage in a fashion similar to that previously described; however, Schwarz suggested using substantially less gain as the outer loop need only correct for the regulation loss associated with the filter and other down-stream transformation elements, such as, a DC-AC inverter, etc.
From the amplified error voltage, Schwarz suggested subtracting the scaled reference voltage and adding a scaled portion of the internal-node switched voltage to develop a voltage which Schwarz suggested integrating. Finally, Schwarz suggested sensing when the level of the integrated voltage reaches a predetermined level to develop a signal for triggering a monostable (one-shot) multi-vibrator which is employed to drive the switch.
Although the Schwarz technique would suggest that full loop gain could be used up to substantially the switching frequency (pulse-train rate), practical implementations substantially limit the achievable performance. Specifically, voltage changes must charge stray circuit capacitances which limit the maximum rate of change thereof imposing delays in the feedback loop. (The delays are undesirable because the maximum available gain of a Type 1 feedback loop is an inverse function of the signal delay within the loop.)
Schwarz discloses an implementation of the previously described technique in the U.S. Pat. No. 3,659,184. The reader may also find of interest the U.S. Pat. Nos. 3,303,405 and 3,311,808, also of Francisc C. Schwarz.